STEREO Observations of Small Transients in the Solar Wind at 1 AU (2007-2014) and Comparison with Near - Earth Results from Wind (1995 - 2014)

The occurrence frequency of small transients (STs) in the solar wind increases dramatically during solar activity minimum. With observations from the Wind spacecraft we could observe STs near Earth over a period of 20 years. Now we also have measurements by the STEREO spacecraft, which observe at 1 AU as they separate from Earth by 22.5 degrees per year. Hence we have an unprecedented opportunity to study the STs at the far side of the Sun at 1 AU, and examine their distribution over solar cycle activity (~ 8 years). We show their properties per year at STEREO, and compare them with Wind's results. The occurrence of STs is found to be anti-correlated with that of ICMEs (STs are frequently observed in the solar minimum while the ICMEs are fewer). We then model some flux-rope type STs by non-force free models, both analytical and numerical ones. Most of the fitting results show that the cross-section of the small flux ropes tends to be elliptical. Finally we show how their orientations are distributed at 1 AU.